The metal halide lamp which began in the early sixties and contains mercury and metal halides for the fill has become one of the most useful and versatile light sources. It has good color rendition, high efficiency usually exceeding 100 lumens per watt in the larger sizes, and relatively long useful life in excess of 10,000 hours.
In the manufacture of these lamps, dispensing the required quantity of metal halide salt into the lamp envelope presents a problem because the inclusion of hydrogen or oxygen in any form is highly detrimental. Oxygen present within the envelope may oxidize metals such as tungsten which make up the lamp electrodes and the resulting metal oxide condenses on the envelope wall and reduces light transmission. When hydrogen is also present, a cyclic action may take place wherein the hydrogen reduces metal oxide on the walls back to metal, freeing the oxygen to attack other electrode metal. The cyclic action continues resulting in rapid erosion of the electrodes and darkening of the walls to the point where useful life is terminated. In order to prevent or alleviate the foregoing possibilities, it is necessary to use for the fill highly purified materials, for instance metal halide salts wherein impurities such as hydrogen or oxygen are held down to a few parts per million, for instance less than 20 ppm. In addition, since many of the metal halides are highly hygroscopic, it is necessary to handle them in such fashion as to minimize the absorption of moisture or impurities from the atmosphere.
U.S. Pat. No. 3,676,534--Anderson, Process Relating to Ultrapure Metal Halide Particles, 1972, describes a vacuum shot tower technique for preparing purified metal halides as spheroidal particles of controlled size. While such particles have been successfully used in lamp making, the cost of preparing them is relatively high. Also the process for preparing them suffers from lack of flexibility in the size of particle produced.